The present invention relates generally to diagnostic imaging and, more particularly, to a method and system of thermoacoustic computed tomography (TCT) such that unmeasured TCT data is determined from measured TCT data.
It is generally well known that wave propagation and integral geometry are the physical and mathematical underpinnings of most diagnostic imaging modalities. To date, most of these standard modalities have been predicated upon the measurement of the same type of output energy as was input to the system. For example, ultrasound diagnostic systems transmit and receive ultrasonic waves and, from those ultrasonic waves, are capable of generating a diagnostic image. CT systems are predicated upon the transmission and reception of x-ray or gamma ray radiation. In conventional CT systems, x-rays are projected toward an imaging subject and the attenuation of those x-rays caused by the subject is measured and processed to reconstruct a diagnostically valuable and probative image of the subject. Recently, however, hybrid imaging or diagnostic systems have been developed along with associated imaging techniques whereupon the measured output energy is different in form and type from the energy input to the system.
For instance, thermoacoustic tomography (TCT) is predicated upon and uses radio frequency (RF) energy projected at an imaging subject and measures emitted ultrasonic waves resulting from the application of the RF energy. Near infrared radiation is also non-ionizing and may also be used to heat tissue. TCT imaging involves the measurement of ultrasonic signals that are induced in the tissue of a subject whenever pulsed or continuous application of radiation is absorbed within the tissue, and the detection of resulting ultrasonic signals with transducers placed on or outside the imaging subject. More particularly, the ultrasonic transducers placed about the subject detect shock waves that are created in tissue when RF energy is absorbed and cause a heating and expansion of tissue. For example, it is known that cancerous masses absorb more RF energy than healthy tissue. As such, cancerous masses preferentially absorb RF energy, heat, and expand more quickly than neighboring healthy tissue thereby creating a shock wave which, when detected by an ultrasonic transducer, allows for detection of, or contrast between, cancerous or abnormal tissues and healthy tissues. Therefore, assuming a constant sound speed, the sound or ultrasonic waves, detected at any point in time after application of the RF energy, are generated by inclusions or abnormal masses lying on a sphere of radius centered at a particular transducer. This is particularly illustrated in FIG. 1.
Turning to FIG. 1, a sphere 10 is shown with an imaging object 12 centrally placed therein. Placed at various positions along a lower portion 11 of the sphere are transducers 14 that will be used to detect or receive ultrasonic waves created within the imaging object as a result of the application of electromagnetic or RF energy. Further shown in FIG. 1 is an inadmissible transducer location generally referenced 16. The transducer location 16 is inadmissible given the imaging object's positioning within sphere 10. That is, in the example of application of TCT imaging to image a breast, the patient is placed face down on an imaging table such that the patient's breast is positioned within sphere portion 11. Therefore, it is clear that the breast is positioned within the lower hemisphere 11 of sphere 10 whereas the top portion 13 of sphere 10, which is shown in phantom, is occupied by portions from the patient on which data is be collected. As such, it is not possible to use or place a transducer at the referenced location 16 to collect data from object 12.
With TCT imaging, inversion of a generalized Radon transform is required because integrals of the tissue's RF absorption coefficient are measured over surfaces of the sphere. The data measured at the admissible transducer locations 14 is therefore an integral of the RF absorbtivity function over data arcs 15, 17, 18 in dimension N=3, given the three illustrated admissible transducer locations shown in FIG. 1. In other words, integrals along the arcs that would be associated with transducer location 16 cannot be directly measured with a transducer at location 16. The non-measurable arcs associated with transducer location 16 are shown as dashed arcs 19.
The inability to directly measure the data associated with a dedicated transducer at location 16 is problematic for a number of reasons. For instance, a lesion, cancerous mass, or other tissue abnormality that is the target of the diagnostic procedure may be within the realm of detection associated with the inadmissible transducer location 16 and, as such, is not sufficiently detected with transducer's position at the admissible transducer locations. In addition, current TCT imaging reconstruction techniques use filtering and backprojecting of only the data that is directly measured. As a result, using only the measured data may cause low frequency shading in the reconstructed image for relatively uniform imaging objects. Additionally, objects with high frequency content may suffer more severe artifacts when data or transducer locations go unmeasured.
It would therefore be desirable to design a method and system of TCT imaging whereupon data from otherwise directly unmeasurable transducer locations may be determined to improve not only image quality, but also sensitivity of a TCT data acquisition to an imaging object.